Wednesday, July 23, 2008
 
     
About Us |  Branch Locations |  Specialty Departments  |  Corporate Connection | Careers
Vendors and Products |  Discontinued Progress Fixtures |  Formulas |  Credit Application
     
*Common Formulas For Electrical Work
E = Voltage -- I = Amps -- W = Watts
PF = Power Factor -- Eff = Efficiency -- HP = Horsepower

*AC/DC Formulas
To Find Direct Current AC / 1phase
115v or 120v 		AC / 1phase
208,230, or 240v 		AC 3 phase
All Voltages
Amps when
Horsepower
is Known 		HP x 746
E x Eff 		HP x 746
E x Eff X PF 		HP x 746
E x Eff x PF 		HP x 746
1.73 x E x Eff x PF
Amps when
Kilowatts
is known 		kW x 1000
E 		kW x 1000
E x PF 		kW x 1000
E x PF 		kW x 1000
1.73 x E x PF
Amps when
kVA is known 		  kVA x 1000
E 		kVA x 1000
E 		kVA x 1000
1.73 x E
Kilowatts I x E
1000 		I x E x PF
1000 		I x E x PF
1000 		I x E x 1.73 PF
1000
Kilovolt-Amps   I x E
1000 		I x E
1000 		I x E x 1.73
1000
Horsepower
(output) 		I x E x Eff
746 		I x E x Eff x PF
746 		I x E x Eff x PF
746 		I x E x Eff x 1.73 x PF
746

*Three Phase Values
For 208 volts x 1.732, use 360
For 230 volts x 1.732, use 398
For 240 volts x 1.732, use 416
For 440 volts x 1.732, use 762
For 460 volts x 1.732, use 797
For 480 Volts x 1.732, use 831

*AC Efficiency and Power Factor Formulas
To Find Single Phase Three Phase
Efficiency 746 x HP
E x I x PF 		746 x HP
E x I x PF x 1.732
Power Factor Input Watts
V x A 		Input Watts
E x I x 1.732

*Power - DC Circuits
Watts = E xI
Amps = W / E

*Ohm's Law / Power Formulas
P = Watts
I = Amps
R = Ohms
E = Volts 

OHMS Law Capacitance in Microfarads at 60 HZ.
Ohms = Volts/Amps (R=E/I)
Amps = Volts/Ohms (I=E/R)
Volts = Amps x Ohms (E+IR)		 Capacitance = 2650 x Amperes
Volts

Capacitance = 2.65 x kVAR
(Volts) squared

*Voltage Drop Formulas
Single Phase
(2 or 3 wire) 		VD = 2 x K x I x L
CM 		K = ohms per mil foot 
Copper = 12.9 at 75°)
(Alum = 21.2 at 75°)
Note: K value changes
with temperature.
L = Length of conductor in feet
I = Current in conductor (amperes)
CM = Circular mil area of conductor
CM= 2K x L x I
VD
Three Phase VD= 1.73 x K x I x L
CM
CM= 1.73 x K x L x I
VD

To Obtain Single Phase (ac) Three Phase (ac)
Kilowatts (V x I x pf) / 1000 (1.732 x V x I x pf) / 1000
kVa (V x I) / 1000 (1.732 x V x I) / 1000
Horsepower required when generator kW unknown
(if generator efficiency is unknown, use 0.93)		 (kW) / (0.746 x Efficiency) (kW) / (0.746 x Efficiency)
kW input when motor hp known (if motor efficiency unknown, use 0.85 X hp) (hp x 0.746) / (Efficiency) (hp x 0.746) / (Efficiency)
Amperes when motor hp known (hp x 0.746) / (V x pf x Efficiency) (hp x 0.746) / (1.732 x V x pf x Efficiency)
Amperes when kW known (kW x 1000) / (V x pf) (kW x 1000) / (1.732 x V x pf)
Amperes when kVa known (kVa x 1000) / (V) (kVa x 1000) / (1.732 x V)

*Formulas For Electrical Motors:
To Find: Direct
Current		 Single Phase Three Phase
Horse Power E x I x EFF
 746		 E x I x EFF x PE
 746		 1.732 x E x I x EFF x PF
 746
Current 746 x HP
 E x EFF		 746 x HP
 E x EFF x PF		 746 x HP
1.732 x E x EFF x PF
Efficiency 746 x HP
E x I		 746 x HP
E x I x PF		 746 x HP
1.732 x E x I x PF
Power Factor --------- Input Watts
E x I		 Input Watts
1.732 x E x I

*Formulas For Electrical Circuits:
To Find: Direct Current Single Phase Three Phase
Amperes Watts
Volts		 Watts
Volts x Power Factor		 Watts
1.732 x Volts x Power Factor
Volt-Amperes -------------------- Volts x Amperes 1.732 x Volts x Amperes
Watts Volts x Amperes Volts x Amperes
x Power Factor		 1.732 x Volts x Amperes
x Power Factor

*Horsepower
HP = Torque (in - lbs.) x RPM
63,025
HP = Torque (ft - lbs.) x RPM
5,252
HP = Volts x Amperes x Power Factor x Efficiency
746
Power Factor = Volts x Amperes=Watts
KVA = Volts x Amperes x 1.732
1000
KVA = KW x Power Factor
   
Speed of AC Motors
Synchronous = Hertz x 120
Poles
Percent Slip =
Synchronous RPM - Full Load RPM
Synchronous RPM 		x 100

*Rules of Thumb
 
1HP = 746 Watts
1 KW = 1.34 HP
1HP @ 3450 RPM = 1.5 Ft-lbs Torque
1HP @ 1750 RPM = 3 Ft-lbs Torque
1HP @ 1170 RPM = 4.5 Ft-lbs Torque
1HP @ 875 RPM = 6.0 Ft-lbs Torque
3 Phase Motor @ 550V = 1 AMP/HP
3 Phase Motor @ 460V = 1.25 AMP/HP
3 Phase Motor @ 230V = 2.5 AMP/HP

NEMA Code Letters for Locked-Rotor KVA
The letter designations for locked-rotor kVA per horsepower as measured at full voltage and rated frequency are as follows.
Letter
Designation		 KVA per
Horsepower*
A
B
C		 0.0 - 3.15
3.15 - 3.55
3.55 - 4.0
D
E
F
G		 4.0 - 4.5
4.5 - 5.0
5.0 - 5.6
5.6 - 6.3
H
J		 6.3 - 7.1
7.1 - 8.0
K
L
M		 8.0 - 9.0
9.0 - 10.0
10.0 - 11.2
N
P
R
S		 11.2 - 12.5
12.5 - 14.0
14.0 - 16.0
16.0 - 18.0
T
u
V		 18.0 - 20.0
20.0 - 22.4
22.4 - & up

*Allowable Starts and Starting Intervals
Design A and B Motors
HP 2 Pole 4 Pole 6 Pole
A B C A B C A B C
1
1.5
2
3		 15
12.9
11.5
9.9		 1.2
1.8
2.4
3.5		 75
76
77
80		 30
25.7
23
19.8		 5.8
8.6
11
17		 38
38
39
40		 34
29.1
26.1
22.4		 15
23
30
44		 33
34
35
36
5
7.5
10
15		 8.1
7.0
6.2
5.4		 5.7
8.3
11
16		 83
88
92
100		 16.3
13.9
12.5
10.7		 27
39
51
75		 42
44
46
50		 18.4
15.8
14.2
12.1		 71
104
137
200		 37
39
41
44
20
25
30
40		 4.8
4.4
4.1
3.7		 21
26
31
40		 110
115
120
130		 9.6
8.8
8.2
7.4		 99
122
144
189		 55
58
60
65		 10.9
10.0
9.3
8.4		 262
324
384
503		 48
51
53
57
50
60
75
100		 3.4
3.2
2.9
2.6		 49
58
71
92		 145
170
180
220		 6.8
6.3
5.8
5.2		 232
275
338
441		 72
85
90
110		 7.7
7.2
6.6
5.9		 620
735
904
1181		 64
75
79
97
125
150
200
250		 2.4
2.2
2.0
1.8		 113
133
172
210		 275
320
600
1000		 4.8
4.5
4.0
3.7		 542
640
831
1017		 140
160
300
500		 5.4
5.1
4.5
4.2		 1452
1719
2238
2744		 120
140
265
440

Where: A = Maximum number of starts per hour.
B = Maximum product of starts per hour times load Wk².
C = Minimum rest or off time in seconds between starts.
 
Allowable starts per hour is the lesser of (1) A or (2) B divided by the load Wk², i.e.,
Starts per hour < A or B Load Wk² , whichever is less.
 
 
Example:
 25 hp, 4 pole, load Wk² = 50 From Table, A = 8.8, B = 122.
Starts per hour = 122
50		 = 2.44
 
Calculated value is less than A. Therefore allowable starts/hour = 2.44. Note: Table is based on following conditions:
 
  1. Applied voltage and frequency in accordance with NEMA Standards MG 1-12.44.
  2. During the accelerating period, the connected load torque is equal to or less than a torque which varies as the square of the speed and is equal to 100 percent of rated torque at rated speed.
  3. External load Wk² equal to or less than the values listed in Column B.
 
For other conditions, consult the manufacturer. Reference: NEMA Standards MG 10

*Starting Characteristics of Squirrel Cage Induction Motors
Starting Method Voltage
at Motor		 Line
Current		 Motor
Torque
Full-Voltage Value 100 100 100
Autotransformer
80% tap
65% tap
50% tap
80
65
50
64*
42*
25*
64
42
25
Primary Resistor Typical Rating 80 80 64
Primary Reactor
80% tap
65% tap
50% tap
80
65
50
80
65
50
64
42
25
Series-Parallel 100 25 25
WYE-DELTA 100 33 33
Part-Winding (½ - ½)
2 to 12 Poles
14 and more Poles
100
100
70
50
50
50
Soft start is also available using solid-state controls. Consult manufacturer for voltage, current and torque rating. *Autotransformer magnetizing current not included.
Magnetizing current is usually less than 25 percent of motor full-load current.

*NEMA Size Starters for Three-Phase Motors
NEMA
Size		 Maximum Horsepower - Polyphase Motors
Full-Voltage
Starting		 Auto-Transformer
Starting		 Part-Winding
Starting		 WYE-DELTA
Starting
200V 230V 460V
575V		 200V 230V 460V
575V		 200V 230V 460V
575V		 200V 230V 460V
575V
00
0
1
2
3
4
5
6
7
8
9		 1½
3
7½
10
25
40
75
150
- -
- -
- -		 1½
3
7½
15
30
50
100
200
300
450
800		 2
5
10
25
50
100
200
400
600
900
1600		 - -
- -
7½
10
25
40
75
150
- -
- -
- -		 - -
- -
7½
15
30
50
100
200
300
450
800		 - -
- -
10
25
50
100
200
400
600
900
1600		 - -
- -
10
20
40
75
150
- -
- -
- -
- -		 - -
- -
10
25
50
75
150
300
450
700
2600		 - -
- -
15
40
75
150
350
600
900
1400
2600		 - -
- -
10
20
40
60
150
300
500
750
1500		 - -
- -
10
25
50
75
150
350
500
800
1500		 - -
- -
15
40
75
150
300
700
1000
1500
3000

*Starter Enclosures
Type NEMA Enclosure
1 General Purpose - Indoor
2 Driproof - Indoor
3 Dusttight, Raintight, Sleettight - Outdoor
3R Raintight, Sleet Resistant - Outdoor
3S Dusttight, Raintight, Sleettight - Outdoor
4 Watertight, Dusttight, Sleet Resistant-Indoor & Outdoor
4X Watertight, Dusttight, Corrosion-Resistant - Indoor & Outdoor
5 Dusttight, Drip-Proof--Indoor
6 Occasionally Submersible, Watertight, Sleet Resistant - Indoor & Outdoor
6P Watertight, Sleet Resistant-Prolonged Submersion - Indoor & Outdoor
12 Dusttight and Driptight - Indoor
12K Dusttight and Driptight, with Knockouts - Indoor
13 Oiltight and Dusttight - Indoor Hazardous Location Starters
7 Class I, Group A, B, C or D Hazardous Locations - Indoor
8 Class I, Group A, B, C or D Hazardous Location - Indoor & Outdoor
9 Class II, Group E, F or G Hazardous Locations - Indoor
10 Requirements of Mine Safety and Health Administration

*Conversion of NEMA Type Numbers to IEC Classification Designations
(Cannot be used to convert IEC Classification Designations to NEMA Type Numbers)
NEMA
Enclosure
Type Number		 IEC Enclosure
Classification
Designation
1
2
3
3R
3S
4 and 4X
5
6 and 6P
12 and 12K
13		 IP10
IP11
IP54
IP14
IP54
IP56
IP52
IP67
IP52
IP54
Note: This comparison is based on tests specified in IEC Publication 529.
Reference: Information in the above tables is based on NEMA Standard 250-1991.

*Motor Application Formulas Output
Horsepower = Torque (lb. ft.) x RPM
5250		 Kilowatts = Torque (N·m) x RPM
9550
Torque (lb. ft.) = Horsepower x 5250
RPM		 Torque (N·m) = Kilowatts x 9550
RPM

*Speed - AC Machinery
Synchronous RPM = 120 x Frequency (Hz)
Number of Poles 
Percent Slip = Synchronous RPM = Full-Load RPM
Synchronous RPM		 x100

*Time for Motor to Reach Operating Speed (in Seconds)
Seconds = Wk2 (lb. ft.2) x Speed Change (RPM)
308 x Avg. Accelerating Torque (lb. ft.)
Wk2 = Inertia of Rotor + Inertia of Load x Load RPM2
Motor RPM2
Average Accelerating Torque = [(FLT + BDT)/2] + BDT + LRT
3
Where: BDT = Breakdown Torque
FLT = Full-Load Torque
LRT = Locked-Rotor Torque

*Shaft Stress
Shaft Stress (psi) = HP x 321.000
RPM x D3
Shaft Stress (kg/mm2) = KW x 4.96 x 106
RPM x D3
Where:
 D = Shaft Diameter (in or mm)
HP = Motor Output
KW = Motor Output
psi = Pounds Per Square Inch
RPM = Revolutions Per Minute

*Centrifugal Applications
Affinity Laws Fans and Blowers
Flow1
Flow2		 = RPM1
RPM2		 HP = CFM x PSF
33000 x Efficiency of Fan
Pres1
Pres2		 = (RPM1 )2
(RPM 2) 2		 HP = CFM x PIW
6343 x Efficiency of Fan
HP1
HP2		 = (RPM1 )3
(RPM 2) 3		 HP = CFM x PSI
229 x Efficiency of Fan
Where:


 Pres = Pressure
RPM = Revolutions Per Minute

 CFM = Cubic Feet Per Minute
PIW = Inches of Water Gauge
PSF = Pound Per Square Foot
PSI = Pounds Per Square Inch

*Pumps
HP = GPM x FT x Specific Gravity
3960 x Efficiency of Pump		 Volume of Liquid in a Tank

Gallons = 5.875 x D2 x H 1 gallon (uS) of water weighs 8.35 lb. Specific gravity of water = 1.0
HP = GPM x PSI x Specific Gravity
1713 x Efficiency of Pump
Where:

 FT = Head in feet*
GPM = Gallons per minute
PSI = Pounds per square inch
*Head in feet = 2.31 x pounds per square inch gravity. Where: D = Tank diameter (ft)
H = Height of liquid (ft)

*Be Sure To Read Our Legal Disclaimer By Clicking Here
 
 
 
image   image   image   image   image
 
Certain links in this Site connect to other Web Sites maintained by third parties over whom Ralph Pill Electric Supply Co. has no control.   Ralph Pill Electric Supply Co. makes no representations as to the accuracy or any other aspect of information contained in other Web Sites.
 
Copyright © Ralph Pill Electric. 2004-2008. All Rights Reserved.
RalphPill.com And the Ralph Pill Logo Are Trademarks Of Ralph Pill Electric Supply Co.
Send Comments to Information.Technology@RalphPill.com
Click here to view our Legal Disclaimer